The present invention relates to a brazing flux and a production process thereof and, more specifically, to a brazing flux suitable for aluminum and aluminum alloy of a heat exchanger made of aluminum, in particular, used in a car cooler and a production process of the flux. 2. Description of prior art
As is well known, brazing of aluminum or aluminum alloy (hereinafter referred collectively to as "aluminum material") is performed by using a flux. Recently, corrosion resistant fluoride flux has been increasingly used as the brazing flux. The flux of this kind has a melting point lower than eutectic aluminum-silicon alloys (i.e., composites having their melting points in the range of 520.degree. to 620.degree. C.) and has an advantage of being very active and effective in the aspect of accelerating wetting property of brazing filler metal in a melting state by breaking and removing oxide film formed on the aluminum matrix and brazing surface or by reducing interfacial tension. Furthermore, the flux of this kind has another great advantage of being not necessary to be removed after completing the welding because the flux is transformed into an inactive compound having no hygroscopic property, solubility and reactivity after cooling and solidification.
It has been a recent trend to require brazing flux to exhibit following higher performances corresponding to fine and complicated structure of aluminum material:
(a) As the temperature control of brazing is more difficult with increasing complexity in structure of aluminum material to be brazed, a flux of further low melting point is required; and
(b) For the purpose of evenly applying the flux to the brazing surface of an object of complicated structure, suspension stability of flux particles in water is fatally important.
In other words, in the steps of causing a flux to suspend in water, dipping a structure to be brazed in water, picking it up, and forming a flux layer on the brazing surface, it is essential to disperse flux particles evenly in the liquid without sedimentation from the viewpoint of perfect brazing.
As one of the fluoride fluxes of this kind, there is a flux of fluoroaluminate of alkali metal, and, for example, following methods of producing this flux were known:
(1) U.S. Pat. No. 3,951,328 discloses a method comprising the steps of mixing aluminum fluoride and potassium fluoride, melting the mixture by heating, and grinding it into fine particles after cooling and solidification thereof.
(2) G.B. Pat. No. 1,055,914 discloses a reaction method comprising the step of kneading fine particles of aluminum fluoride and potassium fluoride with water.
(3) Japanese Laid-Open Patent Publications (unexamined) Nos. 60-203395 and 60-204616 disclose a reaction method comprising the steps of dissolving aluminum hydroxide into hydrofluoric acid, and causing potassium hydroxide solution to act thereon in the temperature range of 30.degree. to 100.degree. C. and under acidity condition (pH=4 and below).
(4) J. Am Ceram. Soc. Vol.49, P.63l (1961) discloses a method comprising the steps of adding alkali metal fluoride to HF solution of AlF.sub.3, and precipitating K.sub.2 AlF.sub.5 H.sub.2 O.
(5) "Inorganic and Theoretical Chemistry", Vol. 5, P.306 (1961) discloses a method for obtaining K.sub.2 AlF.sub.5 comprising the step of causing aluminum hydroxide to act on aqueous solution of potassium bifluoride.
(6) Japanese Laid-Open Patent Publication (unexamined) No. 61-162295 discloses a method for obtaining a complex compound called cesium fluoroaluminate composed of AlF.sub.3 and CsF, and mole ratio of AlF.sub.3 /CsF is in the range of 67/33 to 26/74.
In both methods (1) and (2) mentioned above, precomposed materials fluoride are used and coupled by melting process or wet kneading process
The method (1) has a disadvantage of requiring processes of considerably consuming energy and grinding the solid mixture produced of the melted materials into fine particles of 150 to 200 micron meter, before putting into practical use.
The method (2) has a disadvantage of delay in the reaction with potassium fluoride and residual of materials left without reaction, unless the aluminum fluoride material is ground into sufficiently fine particles. In spite of necessity of such grinding process, size of the fine particles obtained by both methods (1) and (2) mentioned above is as large as several hundred micron meter, and therefore the fine particles of this size do not suspend in water but separate and precipitate. It was reported that the melting point of this flux was 560.degree. C. in both methods (1) and (2).
The method (3) discloses a production process by chemical reaction, and in which no grinding step is required. But the product obtained is a crystalline compound having no property of suspension in water. It was reported that the melting point of this flux was also 560.degree. C.
The method (4) discloses that K.sub.2 AlF.sub.5 .multidot.H.sub.2 O is produced by chemical reaction, and the product thus obtained is also a crystalline compound having no property of suspension in water. It was recognized as a result of measurement that the melting point of the flux was 588.degree. C.
The method (5) merely reports that K.sub.2 AlF.sub.5 is produced by the reaction between potassium bifluoride and aluminum hydroxide.
With regard to cesium fluoroaluminate composites such as Cs.sub.3 AlF.sub.6, Cs.sub.3 AlF.sub.5 H.sub.2 O, CsAlF.sub.4 or the like obtained by the method (6), it was found that the melting points thereof were as low as 450.degree. C., but that suspension stability in water was poor eventually resulting in separation and sedimentation. Moreover, the compound of fluoroaluminate of alkali metal produced by the conventional method was a crystalline compound of MAlF.sub.4, M.sub.2 AlF.sub.5. H.sub.2 O and/or M.sub.3 AlF.sub.6 having a property of sedimentation, and as a result fluxes obtained by the conventional methods are the one having poor suspension stability in water.